利用理論計算探討釕金屬之Innocent和Non-innocent ligand在電催化還原二氧碳反應機制上的差異

全文

(1)國立台灣師範大學化學系碩士論文. 指導教授：蔡明剛博士. 利用理論計算探討釕金屬之 Innocent 和 Non-innocent ligand 在電催化還原二氧化碳反應機制上的差異. Theoretical CO2 Reduction Mechanism by RuPolypyridyl Complexes: An Innocent and Non-Innocent Comparison. 研究生：鄒語騏. 中華民國一百零五年七月 I.

(2) 謝誌還記得大學時期所接觸到的都是實驗部分，對於理論計算這方面不甚了解。隨著研究所進入理論計算實驗室後，才發現，原來可以從另一個角度看化學。首先最要感謝的是蔡明剛老師在這四年來不厭其煩的教導，從最基礎的程式操作，到後來實驗探討，一點一滴的拉拔我們成長。在報告文獻時告訴我，不可以通通相信文獻，有時候還需要靠自己判斷力，培養我們獨立思考，以及發現問題時要如何解決。在實驗探討的部分，偶爾會覺得數據不太合理，因此感到恐慌，但老師總是鼓勵我們，讓我覺得又看見希望。其次要謝謝口試委員江志強、李祐慈、陳韋甫老師，謝謝你們的建議及指導，讓我更清楚研究還不足的地方，以及對於論文的流暢性、嚴謹性，使其更加完善。回想起剛進入實驗室時，與實驗組不同的是，我們的實驗室很大、好多台電腦，當時實驗室成員皆為男性，僅有兩位女性書儀、亮君，景象非常特別。學長們也都非常幽默、好相處，非常感謝我的學長們，弼豊學長很有耐心的教導我，使我在研究上能更快的進入主軸；長誌學長的幽默風趣，為實驗室更添增幾分生氣；鈞普學長的出遊紀實，過了許久再看還是能憶起當日情景；酷酷的正明學長，還有淡定的財興學長，偶爾一開口滿是笑料。謝謝在研究所期間陪伴、支持我的同學及學弟妹們:實驗室的大神哲銘、英文很好的學謙、溫柔婉約的書儀、霸氣的堯舜、善良體貼的家慧、獨立的郁潔、娃娃音的文霜、攝影很強的侑得、羽球很厲害的振成，還有一起口試奮鬥的好夥伴雨柔、佳臻、怡碩、相均，此外還要謝謝許多的朋友們，有了你們，使我的碩士生活更加歡樂、精彩。時光匆匆，在念研究所以及修教程的四年裡充滿了許多回憶，最後我要大聲說：我畢業了！鄒語騏. 謹誌. 中華民國 105 年 7 月 II.

(3) Table of contents Table of contents .................................................................................... III List of Figures ........................................................................................... V List of Schemes ....................................................................................... VI List of Tables..........................................................................................VII 中文摘要............................................................................................... VIII Abstract ................................................................................................... IX Chapter 1 Introduction.............................................................................1 1-1. Background ........................................................................................................... 1 1-2. Electrocatalytic reduction ..................................................................................... 2 1-3. Proton-coupled electron transfer (PCET) ............................................................. 3 1-4. Ruthenium complexes in electrocatalytic reduction ............................................. 4 1-5. A series of investigations on CO2 reduction by Koji Tanaka ................................ 6 1-6. Non-innocent ligand ............................................................................................. 8. Chapter 2 Computational Details............................................................9 2-1. The computation method by Truhlar .................................................................... 9 2-2. The computation method36 ................................................................................... 9 2-2-1. Single-Point Energy .................................................................................. 10 2-2-2. Geometry optimization .............................................................................. 10 2-2-3. Frequency .................................................................................................. 11 2-2-4. Solvation model......................................................................................... 11 2-2-5. The calculation of potential, pKa, and free energy in solvent ................... 13. Chapter 3 Result and discussion............................................................16 3-1. Background ......................................................................................................... 16 3-2. The reaction of [Ru(bpy)(trpy)CO]2+ (Innocent Ligand) .................................... 17 3-2-1. [Ru(bpy)(trpy)CO] to [Ru(bpy)(trpy)CHO].............................................. 17 3-2-2. [Ru(bpy)(trpy)CHO] to [Ru(bpy)(trpy)CH2OH] ...................................... 20 3-2-3. [Ru(bpy)(trpy)CH2OH] to [Ru(bpy)(trpy)CO] ......................................... 23 3-2-4. The mechanism of [Ru(bpy)(trpy)CO]2+................................................... 27 3-3. The reaction of [Ru(trpy)(OBQ)CO]2+ (Non-Innocent Ligand) ......................... 28 3-3-1. [Ru(trpy)(OBQ)CO] to [Ru(trpy)(OBQ)CHO] ......................................... 28 III.

(4) 3-3-2. [Ru(trpy)(OBQ)CHO] to [Ru(trpy)(OBQ)CH2OH].................................. 30 3-3-3. [Ru(trpy)(OBQ)CH2OH] to [Ru(trpy)(OBQ)CO] .................................... 33 3-3-4. The mechanism of [Ru(trpy)(OBQ)CO]2+ ................................................ 37 3-4. Charge and Spin distribution .............................................................................. 38 3-4-1. [Ru(bpy)(trpy)CO]2+ ................................................................................. 38 3-4-2. [Ru(trpy)(OBQ)CO]2+ ............................................................................... 41. Chapter 4 Conclusion .............................................................................44 References ................................................................................................45 Appendix ..................................................................................................48. IV.

(5) List of Figures Figure 1-1. Atmospheric CO2 at Mauna Loa Observatory ....................................... 1 Figure 1-2. Electrocatalysis with electron source ..................................................... 2 Figure 1-5. Cyclic voltammograms of [Ru(bpy)(trpy)CO]2+ and plots of amounts for products ................................................................................................................ 6 Figure 2-2-2. Potential Energy Surface .................................................................. 10 Figure 2-2-4. Four type of solvation model ............................................................ 12. V.

(6) List of Schemes Scheme 1-3. PCET .................................................................................................... 3 Scheme 1-5. Mechanism predicted by Tanaka .......................................................... 7 Scheme 1-6. catecholate/o-quinone redox system .................................................... 8 Scheme 2-2-5-1. The thermodynamic cycle of dissociation in gas and solute phase ................................................................................................................................. 13 Scheme 2-2-5-2. Thermodynamic cycle for the proton-coupled reduction of species O to species R between gas phase and liquid phase. .................................. 14 Scheme 3-2-1. Calculated reduction potentials for converting [Ru(bpy)(trpy)CO] to [Ru(bpy)(trpy)CHO] ........................................................................................... 17 Scheme 3-2-2. Calculated reduction potentials for converting [Ru(bpy)(trpy)CHO] to [Ru(bpy)(trpy)CH2OH]. ...................................................................................... 20 Scheme 3-2-3. Calculated reduction potentials for converting [Ru(bpy)(trpy)CH2OH] to [Ru(bpy)(trpy)CO]........................................................ 23 Scheme 3-2-4. The mechanism of [Ru(bpy)(trpy)CO]2+ ........................................ 27 Scheme 3-3-1. Calculated reduction potentials for converting [Ru(trpy)(OBQ)CO] to [Ru(trpy)(OBQ)CHO] ......................................................................................... 28 Scheme 3-3-2. Calculated reduction potentials for converting [Ru(trpy)(OBQ)CHO] to [Ru(trpy)(OBQ)CH2OH]. ............................................... 30 Scheme 3-3-3. Calculated reduction potentials for converting [Ru(trpy)(OBQ)CH2OH] to [Ru(trpy)(OBQ)CO]. .................................................. 33 Scheme 3-3-4. The mechanism of [Ru(trpy)(OBQ)CO]2+...................................... 37. VI.

(7) List of Tables Table 3-2-1. Calculated values of pKa and E0 at the Density Functional Theory Level of [Ru(bpy)(trpy)CO] to [Ru(bpy)(trpy)CHO]. ............................................ 19 Table 3-2-2.Calculated values of pK and E0 at the Density Functional Theory Level of [Ru(bpy)(trpy)CHO] to [Ru(bpy)(trpy)CH2OH]. ............................................... 22 Table 3-2-3-1. Calculated values of pK and E0 at the Density Functional Theory Level of [Ru(bpy)(trpy)CH2OH] to [Ru(bpy)(trpy)COOH]. .................................. 25 Table 3-2-3-2. Calculated values of Gibbs free energy at the Density Functional Theory Level of [Ru(bpy)(trpy)CH2OH] to [Ru(bpy)(trpy)CO2]. .......................... 26 Table 3-3-1. Calculated values of pK and E0 at the Density Functional Theory Level of [Ru(trpy)(OBQ)CO] to [Ru(trpy)(OBQ)CHO]. ....................................... 29 Table 3-3-2. Calculated values of pK and E0 at the Density Functional Theory Level of [Ru(trpy)(OBQ)CHO] to [Ru(trpy)(OBQ)CH2OH]. ................................ 32 Table 3-3-3-1. Calculated values of pK and E0 at the Density Functional Theory Level of [Ru(trpy)(OBQ)CH2OH] to [Ru(trpy)(OBQ)COOH]. ............................. 35 Table 3-3-3-2. Calculated values of Gibbs free energy at the Density Functional Theory Level of [Ru(trpy)(OBQ)CH2OH] to [Ru(trpy)(OBQ)CO2]. ..................... 36 Table 3-4-1-1. Charge distribution of [Ru(bpy)(trpy)CO]2+ ................................... 38 Table 3-4-1-2. Spin distribution of [Ru(bpy)(trpy)CO]2+ ....................................... 39 Table 3-4-2-1. Charge distribution of [Ru(trpy)(OBQ)CO]2+ ................................ 41 Table 3-4-2-2. Spin distribution of [Ru(trpy)(OBQ)CO]2+..................................... 41 Table A-1. [Ru(bpy)(trpy)(CO)]2+........................................................................... 48 Table A-2. [Ru(trpy)(OBQ)(CO)]2+ ........................................................................ 50. VII.

(8) 中文摘要 Koji Tanaka 為少數利用有機金屬錯合物直接將 CO2 進行多電子還原的科學家之一，在 1993 年利用 RuII(bpy)(trpy)(CO), bpy = 2,2'-Bipyridine, tpy = 2,2':6',2”terpyridine,作為催化劑，並可在通入-1.7V 的電壓環境下使 CO2 還原成常見的 CO、 HCOOH，甚至可得到 CH3OH、HC(O)H、以及增加碳-碳鍵的產物 H(O)CCOOH 及 HOCH2COOH，並且在 1994 年推測出一個完整的催化循環。但此催化反應各中間物的詳細訊息在實驗數據上並不明瞭。因此希望藉由理論計算方法分析各步驟的還原電位、pKa 值及自由能去探討其反應的可行性，藉此更加瞭解 Tanaka 所推測的反應機制。此外我們也推測 RuII(tpy)(OBQ)(CO), trpy = 2,2':6',2”-terpyridine, OBQ = ortho-benzoquinone 還原 CO2 的反應機制，並將 RuII(trpy)(OBQ)(CO)視為 non-innocent ligand，而 RuII(bpy)(trpy)(CO)視為 innocent ligand 做比較，探討電子在錯合物上非定域化的程度是否大幅影響催化劑還原 CO2 的能力以及催化途徑的推測。結果很成功地利用兩個催化劑推測完整的催化機制，並可發現鍵結上 non-innocent ligand 的催化劑，可使反應更利於脫除甲醇，但相對地不好和 CO2 進行鍵結。. 關鍵字：二氧化碳、電催化、non-innocent ligand、反應機制、理論計算 VIII.

(9) Abstract Koji Tanaka is one of the few scientists who use organometallic complexes as a catalyst for multi-electron reduction reactions. In 1993, Tanaka reported that RuII(bpy)(trpy)(CO) (bpy = 2,2 '-bipyridine, trpy = 2,2': 6',2"-terpyridine) acts as a catalyst, at −1.7V (vs. NHE), for the reduction of CO2 to generate products of economic value like CO, HCOOH, CH3OH, HC(O)H, and products involving C–C bond formation like H(O)CCOOH and HOCH2COOH. In 1994, Tanaka speculated a complete catalytic cycle; however, the detailed information was not available on each intermediate of the reaction. Therefore, we relied on theoretical calculations to analyze each step of the reaction in terms of reduction potential, pKa values, and free energy, to explore the feasibility of the reaction, thereby understanding more of the reaction mechanism than what Tanaka hypothesized. In addition to RuII(bpy)(trpy)(CO), we speculate that RuII (trpy) (OBQ) (CO) (OBQ = ortho-benzoquinone) can act as a catalyst for the CO2 reduction reaction. RuII(trpy)(OBQ)(CO) as a non-innocent ligand and RuII(bpy)(trpy)(CO) as an innocent ligand were compared to determine whether the degree of electron delocalization between the metal and the ligands significantly affects the catalytic ability for CO2 reduction and to determine the catalytic pathways. We successfully predicted the mechanism of CO2 reduction with two different types of ligands (innocent and non-innocent). A non-innocent ligand has the advantage of reducing CO to form methanol easily on the complex, but the CO2 activation after removal of methanol on ethanol is weaker than innocent ligand system (bpy).. Key words：Carbon dioxide, Electrocatalysis, Non-innocent ligand, Mechanism, and Theoretical computation IX.

(10) Chapter 1 Introduction 1-1. Background With the industrial revolution in the 19th century, carbon dioxide concentrations increased rapidly due to increased use of fossil fuels, resulting in significant increase in the greenhouse effect, which had a major harmful impact on the environment. Nowadays, the rate of increase in atmospheric carbon dioxide is 100 times faster than the ice age. In the 1950s, the carbon dioxide content in the atmosphere increased by 0.7 ppm annually; by the year 2000, the annual rate of increase had gone up to 2.1 ppm. Currently, the annual rate of increase in atmospheric CO2 is 400 ppm, and will easily reach. 450. ppm. in. the. near. Figure 1-1. Atmospheric CO2 at Mauna Loa Observatory 1. future.1.

(11) The increasing concentration of atmospheric carbon dioxide and shortage of fossil fuels are major concerns of recent times. Therefore, the efficient capture2 and conversion of carbon dioxide into other chemicals like methanol or liquid fuel3,4,5 has been a major research goal for modern scientists. Currently, carbon dioxide can be converted to other chemicals by six different techniques6: chemical7,8, photochemical9, electrochemical10,11,12, biological13,14, reforming15, and inorganic16. Electrochemical methods have the advantage of multi-electron reduction of carbon dioxide; however, the effective and accurate control of the position of the electron transfer is a major challenge. To this end, we expect that an organometallic complex can be used as an electron storage device that can help in stepwise multi-electron reduction.. 1-2. Electrocatalytic reduction6,11. Figure 1-2. Electrocatalysis with electron source.24 The catalyst does not usually exist in its crystalline form, but exists in its ionic form. Therefore, the electrode must reduce the catalyst first to its neutral crystalline form, before it can reduce carbon dioxide or other derivatives. The reduction potential of the catalyst should be lower than that of carbon dioxide for carbon dioxide reduction to take place. Moreover, the applied potential should be able to break through 2.

(12) intermediate energy or activation energy barrier at every step of the reaction for the reaction to proceed successfully. However, if the potential of a side reaction is less than the applied energy, the side reaction may also occur. Therefore, to effectively control the reaction pathway is also a challenge.. 1-3. Proton-coupled electron transfer (PCET)17. Scheme 1-3. PCET PCET is a common reaction mechanism for redox reactions.18 It involves the concerted transfer of an electron and a proton to the substrate. PCET is found in many enzymatic. reactions19,. water. oxidation,. carbon. dioxide. reduction20,. and. photosynthesis.21,22 In 1984, Sutin reported a series of investigation on electron transfer in chemistry and biology.23 In 2011, Savéant reported the concerted transfer of an electron and a proton in a single energy transfer process demands attention. For example17: PhOH→ PhO˙+ e–+ H+ The above-mentioned phenol oxidation reaction can proceed via three different 3.

(13) pathways: EPT (proton transfer followed by electron transfer), PET (electron transfer followed by proton transfer), and CPET (concerted proton -electron transfer). 1. EPT: This is a two-step process: first, phenol must be oxidized by losing an electron to generate an intermediate "PhOH˙", which then loses a proton to obtain the final product "PhO˙". 2. PET: This is also a two-step process, opposite to EPT. In this, phenol undergoes a proton transfer first to obtain the intermediate "PhO–", which then undergoes single electron transfer to obtain the final product "PhO˙". The free energy from the starting reactant to the product should be low enough and the pka should be higher than that of the proton acceptor for the reaction will undergo any of the two above-mentioned pathways. 3. CPET: It involves concerted proton-electron transfer, i.e. it involves a singlestep transfer of a proton and an electron without forming any intermediate. This pathway has the advantage of a lower activation energy than both EPT and PET pathways.24 It’s the alternative name of PCET, and we use PCET for this pathway of the following discussion.. 1-4. Ruthenium complexes in electrocatalytic reduction Electrocatalytic reduction of CO2 using metal catalysts that can be separated into three classes based on the ligand types: (1) macrocyclic ligands; (2) bipyridine ligands; and (3) phosphine ligands.6 Scientists usually utilized bipyridine (bpy) ligands coordinated to Ru(II) for catalyzing CO2 reduction. It was reported by Koji Tanaka et al. that Ru(bpy)(CO)22+ and Ru(bpy)(CO)Cl+ could catalyze the conversion of CO2 to CO, HCOO-, and H2. Though the catalysts in electrocatalytic reduction of CO2 have low turnover numbers 4.

(14) and low sensitivity, identifying the key intermediates in the reduction is still useful.25 In 1993, Tanaka used [Ru(bpy)(trpy)CO]2+ as a low-potential catalyst to produce fourelectron products (HC(O)H, H(O)CCOOH) as well as six-electron products (CH3OH, HOCH2COOH).26 Tanaka et al. also predicted the mechanism in 1994.27 In 1991, Hoffman et al. reported ten Ru(II)-diimine complexes of the formula Ru(bpy)3-m2+ z(bpm)m(bpz)z. (bpy =2, 2'-bipyridine, bpm = 2,2'-bipyrimidine, bpz = 2,2'-bipyrazine,. m and z = 0,1,2,3 and m + z ≤ 3) for one-electron reduction; they also reported that for good reduction activity, the catalyst should have the tendency localize electrons on the most easily reduced ligand (bpz > bpm > bpy).28 In 2016, Sascha Ott utilized the bpy ligand of [Ru(tBu3tpy)(bpy)(NCCH3)]2+ with a simple methyl substituent and discovered that having a simple methyl substituent on different positions on the bpy ligand can develop a new mechanistic pathway for reductive disproportionation.29 In 2007, Peter G. Pickup reported that Ru(bpy)2(2-(2-pyridyl)benzothiazole)2+ as a. potential catalyst for CO2 reduction. The acitivities of the benzothiazole and. benzimidazole analogues have a large difference on the CO2 reduction. Bithiazole complexes require multiple reduction to become active. Involving of the -s- as the position for CO2 activation is an important factor in increasing the activities.30 In 2016, Huang et al. reported that a PN3-Ru pincer complex bearing a redoxactive bpy ligand with an aminophosphine is an effective catalyst for CO2 reduction to CO and HCOOH at high FEs with negligible formation of H2 in a H2O/MeCN mixture, and proposed the overall mechanism.31 Under the same conditions, without the PN3-Ru pincer complex, H2 is the major product of CO2 reduction. Two of the most important steps involved are: (1) the bpy performs a ligand-to-metal charge transfer and leads to loss of Cl-, and (2) insertion of CO2 insertion in the complex.. 5.

(15) 1-5. A series of investigations on CO2 reduction by Koji Tanaka. Figure 1-5. Cyclic voltammograms of [Ru(bpy)(trpy)CO]2+ and plots of amounts for products Koji Tanaka is one of the few scientists who use an organometallic complex as a catalyst for direct reduction of CO2 to methanol under an applied negative potential via multi-electron-proton transfer. In 1993, Koji Tanaka used [Ru(bpy)(trpy)CO]2+ as catalyst at −20 oC, using an applied potential of −1.7V for reducing carbon dioxide saturated in a methanol–water mixture (v/v 4:1).26 Carbon dioxide was successfully reduced to produce not only CO and HCOOH but also HCHO and C2H5OH; products involving formation of C–C bonds were also formed.. 6.

(16) Scheme 1-5. Mechanism predicted by Tanaka Tanaka predicted the reaction mechanism of carbon dioxide reduction in 1994.27 [Ru-CO]2+ took up two electrons and one proton to form an intermediate ‘[Ru-CHO]+’. It may obtain a proton to form HCHO in acidic solution or can react with CO2 to form a C2-product H(O)CCOOH. The reduction may continue further to form Ru-CH2OH, and finally CH3OH and HOCH2COOH. On removing the product, the vacancy of the catalyst will be replaced by solvent or CO2, forming Ru-CO2. After dehydration, the catalyst will return to its initial form [Ru-CO]2+ to complete a catalytic cycle. However, every step of the conversion process was not well understood; we propose to use a computational method to obtain more details of every step of the reaction mechanism. 7.

(17) 1-6. Non-innocent ligand. Scheme 1-6. catecholate/o-quinone redox system32 A non-innocent ligand means a ligand that can undergo redox reactions in a catalytic system, e.g. o-quinone (Scheme 1-6). In the example shown, o-quinone can accept two electrons to generate o-semiquinone or catechol,33,34 which helps disperse the charge density of the central metal. Such electron transfers between the metal center and a ligand can maintain a low oxidation state at the metal center. Complexes containing non-innocent ligands undergo active reduction at moderate potentials because of its electrons are delocalized between the metal and the ligands.. 8.

(18) Chapter 2 Computational Details 2-1. The computation method by Truhlar35 We used density functional theory (DFT) with the M11-L functional in Gaussian 09. The 6-311+G-(2df,2p) basis set was used for H, C, N, O, the optimization and frequency calculations were done using the LanL08 pseudopotential for the Ru metal, and the single-point and solvent model MG3S was used for the metal (Ru). The solvent effect in ethanol was considered using the solvation model (SMD) 36.. 2-2. The computation method37 Gaussian software, whose name originates from the Gaussian basis sets in the software, is the most widely used software in quantum chemical calculations. It uses the principles of quantum mechanics to generate wave functions and calculate energy. It can predict gas and liquid conditions, various chemical reactions and properties of molecules, such as molecular structure and energy, vibrational frequencies of molecular systems, infrared and Raman spectroscopy, etc. It can be used to observe intramolecular and intermolecular reactions, the intermediate product, and even the molecular structure of the transition state. We computed the single-point energy, vibrational frequencies, solvent effects, and performed geometry optimization and NPA and population calculations. Some of the calculation results can be used to evaluate reaction free energy potentials and pKa values.. 9.

(19) 2-2-1. Single-Point Energy Calculating single-point energy involves calculating wave function and associated charge density in terms of molecular coordinates. The purpose of a single point energy calculation is to find the charge density for which the total energy functional is minimized.. 2-2-2. Geometry optimization The properties of a molecule are obtained from the representative minimumenergy geometry of the molecule, which is obtained by geometry optimization based on the changing interatomic distances in the molecules and the corresponding energy changes. The energies generated by the changes in molecular geometry are called potential energy surfaces (PES). In terms of mapping coordinates and energy of processes, structures are adjusted in accordance with the PESs until a stable minimum value is obtained, which represents the geometry of a stable molecule.. Figure 2-2-2. Potential Energy Surface As shown in Figure 2-2-2, there are several significant points in PES: global maximum, global minimum, local maximum, local minimum, and saddle point. Global maximum is the highest point of the entire PES, while global minimum is the lowest point, indicating the most stable structure. Local maximum and local minimum, respectively, are the highest and lowest points in a region; the saddle point is crossing point of two high points and two low points, signifying the transition state of these two 10.

(20) low points. In the optimization process, the first derivative of the energy with respect to displacement of the atoms is zero for the global minimum, local minimum, or the saddle point. A second derivative is used to determine whether the obtained result is a saddle point or not. If the obtained frequency is positive, it implies that the associated energy is located on a local minimum; if the frequency value is negative, it implies that the associated energy lies on the saddle point. In addition, when the difference in energy falls in the preset criteria, the program considers that the optimization process has converged.38. 2-2-3. Frequency Single-point energy calculations and geometry optimization ignore the vibrational states of each atom in the molecule, frequencies associated with these vibrational states were analyzed by obtaining the second derivatives of the energy. Frequency analysis for the system also includes thermodynamic analysis in terms of enthalpy and entropy; in the default case, the system calculates the value for a temperature of 298.15 K and pressure of 1 atm using the formula for Gibbs free energy: G = H − TS The resulting of Gibbs free energy plays an important role in this article, and calculated E0 and pKa values are important for the analysis.. 2-2-4. Solvation model The transition state and molecular properties are different between the gas phase and the liquid phase, because the electrostatic field generated by the solution will affect the properties of the molecule. To obtain the molecular properties in liquid phase, a solvation model must be used. The theoretical models of systems in non-aqueous solution are referred to as self11.

(21) consistent reaction field (SCRF) models, where the solvent is described as a continuous reaction field, constituted by the dielectric constant of the solvent. A solute molecule in solvent space will be recalculated in liquid phase in terms of intermolecular forces and reaction field forces. The algorithmic efficiency and results are related to the solvation model, so it is important to choose a suitable solvation model. The following Figure. 2-2-4 displays four types of solvation model.. Figure 2-2-4. Four type of solvation model 1.. Onsager (dipole & sphere) Model: This is the simplest reaction field, where the solute occupies a fixed spherical cavity of a particular radius within the solvent field. An external dipole will induce a dipole in the medium of the molecule, and the solvent dipole produced by the applied electric field will in turn interact with the molecular dipole, leading to net stabilization.. 2.. Tomasi’s Polarized Continuum Model (PCM): The cavity is defined by a series of interconnected atomic spheres, which induce a dipole in the cavity of the solvent reaction field to generate a dipole field according to each atom, leading to electric polarization. This is the most representative model.. 12.

(22) 3.. Isodensity Polarized Continuum Model (IPCM): The cavity is defined as an isodensity surface of the molecule, which is based on the wave functions calculated in the gas phase. The cavity shape is then calculated until it attains a stable value.. 4.. Self-Consistent Isodensity Polarized Continuum Model (SCI-PCM): This procedure solves for the electron density that minimizes the energy, including solvation energy. In contrast to IPCM, SCI-PCM accounts for full coupling between the cavity and the charge density and includes coupling terms.38. 2-2-5. The calculation of potential, pKa, and free energy in solvent In the 1 M standard state, a proton dissociation free energy change in aqueous ∗ solution is given by ∆Gaq (superscript * denotes under 1 M standard state), Which is. calculated using the following thermodynamic cycle:. ∗ ∆Gaq ＝∆G𝑔° + ∆∆Gs∗ + ∆G°→∗. ∆G𝑔° = G𝑔° (𝐴− ) + G𝑔° (𝐻 + ) − G𝑔° (𝐻𝐴) ∆∆Gs∗ = ∆Gs∗ (𝐴− ) + ∆Gs∗ (𝐻 + ) − ∆Gs∗ (𝐻𝐴) Scheme 2-2-5-1. The thermodynamic cycle of dissociation in gas and solute phase39 ∆Gg° can be calculated at 1 atm standard state by initial count or DFT, ∆Gs∗ is the solvation free energy, Gg° (𝐻 + ) is the gas-phase proton free energy, whose value at 298 K is −6.28 kcal/mol as calculated by the Sachur-Tetrode equation39,40,41, ∆Gs∗ (𝐻 + ) value of the aqueous solution at 298 K (−263.98 kcal/mol) was proposed by 13.

(23) Tissandier and coworkers40, ∆G°→∗ is the free energy difference (1.89 kcal/mol) between the standard states in the gas and liquid phases. Due to differences in the thermal correction between gas phase and liquid phase, convergence of the geometry optimization in the liquid phase to obtain ∆Gs∗ is difficult. It cannot be calculated by vibration frequency analysis directly in a continuum solvation model. We do not expect the geometry to change much before and after proton and electron transfer; therefore, the error in the thermal correction term is ignored. The thermal correction term that can be neglected because the differences in thermodynamic cycle are small. ∗ The ∆Gaq value as calculated above, can be used to calculate 𝑝𝐾𝑎 values of the. various intermediates as per the following formula:. 𝑝𝐾𝑎 =. ∗ ∆Gaq 2.303𝑅𝑇. where R is the gas constant and T denotes the temperature in Kelvin.. Scheme 2-2-5-2.. Thermodynamic cycle for the proton-coupled reduction of species. O to species R between gas phase and liquid phase.41 Reduction reaction can be represented by a thermodynamic cycle, as shown in 14.

(24) Scheme 2-2-5-2. R is a reduced acidic species and O is an alkaline oxidized species. The energy of the gas-phase free electrons is −0.006 kcal/mol; if m > 0, it indicates that the reduction reactions involved electron transfer coupled with proton transfer, i.e. the reductions are PCET processes. The free energy change associated with the liquid phase can be obtained from the thermodynamic cycle.. Standard potential can be obtained by the following equation:. ∗ 𝐸𝑂|𝑅 =−. ∗ ∗ (∆𝐺𝑂|𝑅 − 𝑛∆𝐺𝑁𝐻𝐸 ) ∆𝐺0∗ ∗ ∗ − 𝐸𝑆𝐶𝐸 =− − 𝐸𝑆𝐶𝐸 𝑛𝐹 𝑛𝐹. Here, ∆𝐺0∗ is the relative free energy of the standard hydrogen electrode (NHE), and ∗ ∆𝐺𝑁𝐻𝐸 values are 4.28V, and the potential of the standard reference electrode is given ∗ by 𝐸𝑆𝐶𝐸 = +0.24 V vs. NHE.39,43 For m> 0, the standard reduction potential of PCET at. pH = 0 is obtained using the Nernst Equation expressed in terms of pH as follows:. ∗ E = 𝐸𝑂|𝑅 +. 𝑅𝑇 𝑎𝑂 𝑚 ln ( ) − ∙ 0.0591 ∙ 𝑝𝐻 𝑛𝐹 𝑎𝑅 𝑛. where m and n are the numbers of protons and electrons, respectively, R is the gas constant, T is the temperature in Kelvin, F is the Faraday constant, 𝑎𝑖 is a chemical activity (i = O or R); in the reduction half-reaction, the second term is zero.. 15.

(25) Chapter 3 Result and discussion 3-1. Background Theoretical computations in terms of feasibility of reactions are used to confirm the mechanism predicted by Koji Tanaka. Electron and proton transfer processes were studied to determine whether CO2 reduction followed EPT, PET, or PCET mechanism. Potential and pKa data were used discuss the feasibility of the reaction. The energy values obtained via geometry optimization and vibrational frequency analysis were used to confirm the minimum-energy structures. The standard free energy was obtained through a thermodynamic cycle, and was substituted in appropriate formulas to obtain the necessary potential and pKa values. The potential calculated from pH = 0 to pH = 7 at −20oC in C2H5OH/H2O (8:2 v/v) was corrected experimentally. The computed pKa of ethanol is 31. If the pKa value of the complex is larger than 31, it means that the deprotonated form of this complex has an ability to catch a proton from the solvent. The computed potential energy was compared with the external potential applied by Koji Tanaka at −1.70V. If our computed reduction potential energy is less negative than −1.70V, it implies that the reaction is feasible.. 16.

(26) 3-2. The reaction of [Ru(bpy)(trpy)CO]2+ (Innocent Ligand) 3-2-1. [Ru(bpy)(trpy)CO] to [Ru(bpy)(trpy)CHO]. Scheme 3-2-1. Calculated reduction potentials for converting [Ru(bpy)(trpy)CO] to [Ru(bpy)(trpy)CHO]. The green values represent the potentials for ET or PCET, and red values represent the pKa for PT. For [Ru(trpy)(bpy)CO]2+ (referred to as [Ru-CO]2+ in the following discussion) to undergo a PCET process to become [Ru-CHO]2+, a large negative potential of −2.58V and a pKa of −15.9 are required. Hence [Ru-CO]1+ cannot spontaneously accept a proton from a solvent molecule. However, a potential of only −1.05V is needed for direct reduction by an electron. Thus, we assume that [Ru-CO]2+ will undergo an ET process to generate [Ru-CO]1+. There are two possible pathways after [Ru-CO]1+ is formed: one is ET and another is PCET. The required potentials are −1.13V and −0.96V, respectively, with pKa = 15.1. The potential of PCET is lower than that of ET, but the pKa of [Ru(CHO)1+ is lower than ethanol, and thus the reaction does not occur. It likely forms [Ru-CO]0+ by ET rather than form [Ru-CHO]1+ by PCET. When generating [Ru-CO]0+, the reaction still cannot proceed via the PCET process because of the low pKa of [Ru-CHO]0, although the respective potential for the PCET process is −0.97V. The pKa value of this intermediate is 23. We assumed that it will gain one electron via ET and form [Ru-CO]1- rather than undergo a PCET to [Ru17.

(27) CHO]0. The potential of the one-electron reduction is −1.61V. After a series of single-electron reductions to [Ru-CO]1-, [Ru(CO)]1- may be able go to [Ru(CHO)]1- because of the suitable potential (−0.88V) and since pKa (30.6) of [Ru(CHO)]1- is close to 31. The intermediate [Ru-CHO]1- could be generated by the PCET reaction. Due to the redox couple of Ru(CO)]-1/[Ru(CHO)]-1 = -0.88V is larger than [Ru(CO)]0/[Ru(CO)]-1 = -1.61V, the equilibrium of [Ru(CO)]-1 with its neighboring redox species may not be established. [Ru(CO)]-1 could undergo a disproportionation reaction to decompose to [Ru(CO)]0 and [Ru(CHO)]-1; hence the 2e1proton PCET step from [Ru(CO)]0 to [Ru(CHO)]-1 at -1.25V could be the dominate path.. 18.

(28) Reactions. ET Spin. PCET. PT. Spin potential potential pKa. [Ru-(trpy)(bpy)(CO)]2+. (S) + e-. → [Ru-(trpy)(bpy)(CO)]1+. (D) -1.05. [Ru-(trpy)(bpy)(CO)]1+. (D) + e-. → [Ru-(trpy)(bpy)(CO)]0+. (S) -1.25. [Ru-(trpy)(bpy)(CO)]1+. (D) + e-. → [Ru-(trpy)(bpy)(CO)]0+. (T) -1.13. [Ru-(trpy)(bpy)(CO)]0+. (T) + e-. → [Ru-(trpy)(bpy)(CO)]1-. (D) -1.61. [Ru-(trpy)(bpy)(CO)]1-. (D) +. e-. →. [Ru-(trpy)(bpy)(CO)]2-. (S) -2.61. [Ru-(trpy)(bpy)(CO)]1-. (D) + e-. → [Ru-(trpy)(bpy)(CO)]2-. (T) -1.95. [Ru-(trpy)(bpy)(CO)]2-. (T) + e-. → [Ru-(trpy)(bpy)(CO)]3-. (D) -2.3. [Ru-(trpy)(bpy)(CHO)]2+. (D) + e-. → [Ru-(trpy)(bpy)(CHO)]1+. (S) 0.70. [Ru-(trpy)(bpy)(CHO)]2+. (D) + e-. → [Ru-(trpy)(bpy)(CHO)]1+. (T) -0.69. [Ru-(trpy)(bpy)(CHO)]1+. (S) + e-. → [Ru-(trpy)(bpy)(CHO)]0+. (D) -1.14. [Ru-(trpy)(bpy)(CHO)]0+. (D) + e-. → [Ru-(trpy)(bpy)(CHO)]1-. (S) -1.54. [Ru-(trpy)(bpy)(CHO)]0+. (D) + e-. → [Ru-(trpy)(bpy)(CHO)]1-. (T) -1.51. [Ru-(trpy)(bpy)(CHO)]1-. (T) + e-. → [Ru-(trpy)(bpy)(CHO)]2-. (D) -1.93. [Ru-(trpy)(bpy)(CHO)]2-. (D) + e-. → [Ru-(trpy)(bpy)(CHO)]3-. (S) -2.36. [Ru-(trpy)(bpy)(CHO)]2-. (D) + e-. → [Ru-(trpy)(bpy)(CHO)]3-. (T) -2.33. [Ru-(trpy)(bpy)(CO)]2+. (S) + e- + H+ → [Ru-(trpy)(bpy)(CHO)]2+. (D). -2.58. [Ru-(trpy)(bpy)(CO)]1+. (D) + e- + H+ → [Ru-(trpy)(bpy)(CHO)]1+. (S). -0.96. [Ru-(trpy)(bpy)(CO)]1+. (D) + e- + H+ → [Ru-(trpy)(bpy)(CHO)]1+. (T). -2.25. [Ru-(trpy)(bpy)(CO)]0+. (T) + e- + H+ → [Ru-(trpy)(bpy)(CHO)]0+. (D). -0.97. [Ru-(trpy)(bpy)(CO)]1-. (D) + e- + H+ → [Ru-(trpy)(bpy)(CHO)]1-. (S). -0.91. [Ru-(trpy)(bpy)(CO)]1-. (D) + e- + H+ → [Ru-(trpy)(bpy)(CHO)]1-. (T). -0.88. [Ru-(trpy)(bpy)(CO)]2-. (T) + e- + H+ → [Ru-(trpy)(bpy)(CHO)]2-. (D). -0.86. [Ru-(trpy)(bpy)(CO)]3-. (D) + e- + H+ → [Ru-(trpy)(bpy)(CHO)]3-. (S). -0.91. [Ru-(trpy)(bpy)(CO)]3-. (D) + e- + H+ → [Ru-(trpy)(bpy)(CHO)]3-. (T). -0.89. [Ru-(trpy)(bpy)(CO)]1+. (D). + H+ → [Ru-(trpy)(bpy)(CHO)]2+. (D). -15.9. [Ru-(trpy)(bpy)(CO)]0+. (T). + H+ → [Ru-(trpy)(bpy)(CHO)]1+. (S). 15.1. [Ru-(trpy)(bpy)(CO)]0+. (T). + H+ → [Ru-(trpy)(bpy)(CHO)]1+. (T). 15.1. [Ru-(trpy)(bpy)(CO)]1-. (D). + H+ → [Ru-(trpy)(bpy)(CHO)]0+. (D). 23.0. [Ru-(trpy)(bpy)(CO)]2-. (T). + H+ → [Ru-(trpy)(bpy)(CHO)]1-. (S). 30.0. [Ru-(trpy)(bpy)(CO)]2-. (T). + H+ → [Ru-(trpy)(bpy)(CHO)]1-. (T). 30.6. [Ru-(trpy)(bpy)(CO)]3-. (D). + H+ → [Ru-(trpy)(bpy)(CHO)]2-. (D). 36.9. [Ru-(trpy)(bpy)(CO)]4-. (S). + H+ → [Ru-(trpy)(bpy)(CHO)]3-. (T). 44.9. Table 3-2-1. Calculated values of pKa and E0 at the Density Functional Theory Level of [Ru(bpy)(trpy)CO] to [Ru(bpy)(trpy)CHO]. 19.

(29) 3-2-2. [Ru(bpy)(trpy)CHO] to [Ru(bpy)(trpy)CH2OH]. Scheme 3-2-2. Calculated reduction potentials for converting [Ru(bpy)(trpy)CHO] to [Ru(bpy)(trpy)CH2OH]. The green and blue values represent the potentials for ET or PCET, and red values represent the pKa for PT.. In the present study, we followed the criteria of applying external potential 1.70V with higher priority. [Ru-CHO]1- cannot be reduced to [Ru-CHO]2- and this step required -1.93V. There are two possible PCET pathways for [Ru-CHO]1- to form [Ru(CH2O)]-1 or [Ru(CHOH)]-1 the former path forms new CH bond and the later forms new OH bond. Despite the pKa of both [Ru(CH2O)]-1 and [Ru(CHOH)]-1 are not strong enough for the deprotonated form to extract proton from the solvent, the proton tunneling effect or thermal electron redistribution may possibly enhance the proton extract. Thus, the catalytic cycle can go to the next step. The subsequence PCET for both [Ru(CH2O)]-1 and [Ru(CHOH)]-1 is straightforward, since both pKa values are greater than 31, to form [Ru(CH2OH)]-1 species.. 20.

(30) Reactions. ET Spin. PCET. PT. Spin potential potential pKa. [Ru-(trpy)(bpy)(CH2OH)]1+. (S) + e-. → [Ru-(trpy)(bpy)(CH2OH)]0+. (D) -1.24. [Ru-(trpy)(bpy)(CH2OH)]0+. (D) + e-. → [Ru-(trpy)(bpy)(CH2OH)]1-. (S) -1.62. [Ru-(trpy)(bpy)(CH2OH)]0+. (D) + e-. → [Ru-(trpy)(bpy)(CH2OH)]1-. (T) -1.59. [Ru-(trpy)(bpy)(CH2OH)]1-. (T) + e-. → [Ru-(trpy)(bpy)(CH2OH)]2-. (D) -1.98. [Ru-(trpy)(bpy)(CH2OH)]2-. (D) + e-. → [Ru-(trpy)(bpy)(CH2OH)]3-. (S) -2.45. [Ru-(trpy)(bpy)(CH2OH)]2-. (D) + e-. → [Ru-(trpy)(bpy)(CH2OH)]3-. (T) -2.41. [Ru-(trpy)(bpy)(CHO)]1+. (S) + e- + H+ → [Ru-(trpy)(bpy)(CH2O)]1+. (D). -1.42. [Ru-(trpy)(bpy)(CHO)]1+. (S) + e- + H+ → [Ru-(trpy)(bpy)(CHOH)]1+. (D). -1.44. [Ru-(trpy)(bpy)(CHO)]0+. (D) + e- + H+ → [Ru-(trpy)(bpy)(CH2O)]0+. (S). -1.15. [Ru-(trpy)(bpy)(CHO)]0+. (D) + e- + H+ → [Ru-(trpy)(bpy)(CH2O)]0+. (T). -1.40. [Ru-(trpy)(bpy)(CHO)]0+. (D) + e- + H+ → [Ru-(trpy)(bpy)(CHOH)]0+. (S). -1.59. [Ru-(trpy)(bpy)(CHO)]0+. (D) + e- + H+ → [Ru-(trpy)(bpy)(CHOH)]0+. (T). -1.53. [Ru-(trpy)(bpy)(CHO)]1-. (T) + e- + H+ → [Ru-(trpy)(bpy)(CH2O)]1-. (D). -1.45. [Ru-(trpy)(bpy)(CHO)]1-. (T) + e- + H+ → [Ru-(trpy)(bpy)(CHOH)]1-. (D). -1.61. [Ru-(trpy)(bpy)(CHO)]2-. (D) + e- + H+ → [Ru-(trpy)(bpy)(CH2O)]2-. (S). -1.41. [Ru-(trpy)(bpy)(CHO)]2-. (D) + e- + H+ → [Ru-(trpy)(bpy)(CH2O)]2-. (T). -1.31. [Ru-(trpy)(bpy)(CHO)]2-. (D) + e- + H+ → [Ru-(trpy)(bpy)(CHOH)]2-. (S). -1.74. [Ru-(trpy)(bpy)(CHO)]2-. (D) + e- + H+ → [Ru-(trpy)(bpy)(CHOH)]2-. (T). -1.74. [Ru-(trpy)(bpy)(CHO)]3-. (T) + e- + H+ → [Ru-(trpy)(bpy)(CH2O)]3-. (D). -1.48. [Ru-(trpy)(bpy)(CHO)]3-. (T) + e- + H+ → [Ru-(trpy)(bpy)(CHOH)]3-. (D). -1.75. [Ru-(trpy)(bpy)(CH2O)]1+. (D) + e- + H+ → [Ru-(trpy)(bpy)(CH2OH)]1+. (S). -0.01. [Ru-(trpy)(bpy)(CHOH)]1+. (D) + e- + H+ → [Ru-(trpy)(bpy)(CH2OH)]1+. (S). 0.01. [Ru-(trpy)(bpy)(CH2O)]0+. (S) + e- + H+ → [Ru-(trpy)(bpy)(CH2OH)]0+. (D). -0.37. [Ru-(trpy)(bpy)(CHOH)]0+. (T) + e- + H+ → [Ru-(trpy)(bpy)(CH2OH)]0+. (D). 0.00. [Ru-(trpy)(bpy)(CH2O)]1-. (D) + e- + H+ → [Ru-(trpy)(bpy)(CH2OH)]1-. (T). -0.14. [Ru-(trpy)(bpy)(CHOH)]1-. (D) + e- + H+ → [Ru-(trpy)(bpy)(CH2OH)]1-. (T). 0.01. [Ru-(trpy)(bpy)(CH2O)]2-. (T) + e- + H+ → [Ru-(trpy)(bpy)(CH2OH)]2-. (D). -0.33. [Ru-(trpy)(bpy)(CHOH)]2-. (S) + e- + H+ → [Ru-(trpy)(bpy)(CH2OH)]2-. (D). 0.10. [Ru-(trpy)(bpy)(CH2O)]3-. (D) + e- + H+ → [Ru-(trpy)(bpy)(CH2OH)]3-. (T). -0.23. [Ru-(trpy)(bpy)(CHOH)]3-. (D) + e- + H+ → [Ru-(trpy)(bpy)(CH2OH)]3-. (T). 0.04. [Ru-(trpy)(bpy)(CHO)]0+. (D). + H+ → [Ru-(trpy)(bpy)(CH2O)]1+. (D). 7.0. [Ru-(trpy)(bpy)(CHO)]0+. (D). + H+ → [Ru-(trpy)(bpy)(CHOH)]1+. (D). 6.6. 21.

(31) [Ru-(trpy)(bpy)(CHO)]1-. (T). + H+ → [Ru-(trpy)(bpy)(CH2O)]0+. (S). 18.0. [Ru-(trpy)(bpy)(CHO)]1-. (T). + H+ → [Ru-(trpy)(bpy)(CH2O)]0+. (T). 13.5. [Ru-(trpy)(bpy)(CHO)]1-. (T). + H+ → [Ru-(trpy)(bpy)(CHOH)]0+. (S). 10.2. (T). 11.2. H+. →. [Ru-(trpy)(bpy)(CHO)]1-. [Ru-(trpy)(bpy)(CHOH)]0+. (T). +. [Ru-(trpy)(bpy)(CHO)]2-. (D). + H+ → [Ru-(trpy)(bpy)(CH2O)]1-. (D). 19.7. [Ru-(trpy)(bpy)(CHO)]2-. (D). + H+ → [Ru-(trpy)(bpy)(CHOH)]1-. (D). 16.8. [Ru-(trpy)(bpy)(CHO)]3-. (T). + H+ → [Ru-(trpy)(bpy)(CH2O)]2-. (S). 27.2. [Ru-(trpy)(bpy)(CHO)]3-. (T). + H+ → [Ru-(trpy)(bpy)(CH2O)]2-. (T). 29.1. [Ru-(trpy)(bpy)(CHO)]3-. (T). + H+ → [Ru-(trpy)(bpy)(CHOH)]2-. (S). 21.3. [Ru-(trpy)(bpy)(CHO)]3-. (T). + H+ → [Ru-(trpy)(bpy)(CHOH)]2-. (T). 21.2. [Ru-(trpy)(bpy)(CHO)]4-. (D). + H+ → [Ru-(trpy)(bpy)(CH2O)]3-. (D). 32.0. [Ru-(trpy)(bpy)(CHO)]4-. (D). + H+ → [Ru-(trpy)(bpy)(CHOH)]3-. (D). 27.1. [Ru-(trpy)(bpy)(CH2O)]0+. (S). + H+ → [Ru-(trpy)(bpy)(CH2OH)]1+. (S). 27.8. [Ru-(trpy)(bpy)(CHOH)]0+. (T). + H+ → [Ru-(trpy)(bpy)(CH2OH)]1+. (S). 34.7. [Ru-(trpy)(bpy)(CH2O)]1-. (D). + H+ → [Ru-(trpy)(bpy)(CH2OH)]0+. (D). 37.9. [Ru-(trpy)(bpy)(CHOH)]1-. (D). + H+ → [Ru-(trpy)(bpy)(CH2OH)]0+. (D). 40.7. [Ru-(trpy)(bpy)(CH2O)]2-. (T). + H+ → [Ru-(trpy)(bpy)(CH2OH)]1-. (T). 41.0. [Ru-(trpy)(bpy)(CHOH)]2-. (S). + H+ → [Ru-(trpy)(bpy)(CH2OH)]1-. (T). 49.8. [Ru-(trpy)(bpy)(CH2O)]3-. (D). + H+ → [Ru-(trpy)(bpy)(CH2OH)]2-. (D). 50.2. [Ru-(trpy)(bpy)(CHOH)]3-. (D). + H+ → [Ru-(trpy)(bpy)(CH2OH)]2-. (D). 55.1. Table 3-2-2.Calculated values of pK and E0 at the Density Functional Theory Level of [Ru(bpy)(trpy)CHO] to [Ru(bpy)(trpy)CH2OH].. 22.

(32) 3-2-3. [Ru(bpy)(trpy)CH2OH] to [Ru(bpy)(trpy)CO]. Scheme 3-2-3. Calculated reduction potentials for converting [Ru(bpy)(trpy)CH2OH] to [Ru(bpy)(trpy)CO]. The green values represent the potentials for ET or PCET, red values represent the pKa for PT, and blue values represent for the gibbs free energy.. The Gibbs free energy from [Ru-CH2αOHβ]1- to form [Ru-OEt]1- is 3 kcal/mol, and the Gibbs free energy from [Ru-OEt]1- to form [Ru-CO2]0- is -2.48 kcal/mol. It is possible to form [Ru-CO2]0- and [Ru-OEt]1-. We predicted the major reaction is one-electron transfer to [Ru-OEt]1-, because of decreasing the coulomb attraction. The reaction of [Ru-OEt]2- to form [Ru-CO2]1- is considered nonspontaneous, the Gibbs energy is 4.77 kcal/mol. According to scheme 3-2-3., the Gibbs free energy is lower while the more electrons reduced on the complex. It means [Ru-CH2αOHβ] can accept one proton to form CH3OH, which is removed immediately, and the vacancy is replaced by ethanol or CO2 instantaneously with high charge density on complex. The potential of [Ru-CO2]1- is −0.51V and its pKa is 33.7; therefore, it easily 23.

(33) undergoes PCET to form [Ru-COOH]1- or undergoes ET to form [Ru-CO2]2-. In the final step, the dehydration from [Ru-COOH]2- to form [Ru-CO]1-, which carries on performing all the above-mentioned reactions.. 24.

(34) Reactions. ET Spin. PCET. PT. Spin potential potential pKa. [Ru-(trpy)(bpy)(OEt)]0. (D) + e-. → [Ru-(trpy)(bpy)(OEt)]1-. (S) -1.68. [Ru-(trpy)(bpy)(OEt)]0. (D) + e-. → [Ru-(trpy)(bpy)(OEt)]1-. (T) -1.68. [Ru-(trpy)(bpy)(OEt)]1-. (S) + e-. → [Ru-(trpy)(bpy)(OEt)]2-. (D) -1.05. [Ru-(trpy)(bpy)(OEt)]2-. (D) + e-. → [Ru-(trpy)(bpy)(OEt)]3-. (S) -2.37. [Ru-(trpy)(bpy)(OEt)]2-. (D) +. e-. →. [Ru-(trpy)(bpy)(OEt)]3-. (T) -2.35. [Ru-(trpy)(bpy)(CO2)]1+. (D) + e-. → [Ru-(trpy)(bpy)(CO2)]0. (S) -0.88. [Ru-(trpy)(bpy)(CO2)]1+. (D) + e-. → [Ru-(trpy)(bpy)(CO2)]0. (T) -1.24. [Ru-(trpy)(bpy)(CO2)]0. (S) + e-. → [Ru-(trpy)(bpy)(CO2)]1-. (D) -1.36. [Ru-(trpy)(bpy)(CO2)]1-. (D) + e-. → [Ru-(trpy)(bpy)(CO2)]2-. (S) -1.74. [Ru-(trpy)(bpy)(CO2)]1-. (D) + e-. → [Ru-(trpy)(bpy)(CO2)]2-. (T) -1.60. [Ru-(trpy)(bpy)(COOH)]1+. (S) + e-. → [Ru-(trpy)(bpy)(COOH)]0. (D) -1.26. →. [Ru-(trpy)(bpy)(COOH)]1-. (S) -1.54. [Ru-(trpy)(bpy)(COOH)]0. e-. (D) +. [Ru-(trpy)(bpy)(COOH)]0. (D) + e-. → [Ru-(trpy)(bpy)(COOH)]1-. (T) -1.51. [Ru-(trpy)(bpy)(COOH)]1-. (T) + e-. → [Ru-(trpy)(bpy)(COOH)]2-. (D) -1.92. [Ru-(trpy)(bpy)(CO2)]1+. (D) + e- + H+ → [Ru-(trpy)(bpy)(COOH)]1+. (S). -0.02. [Ru-(trpy)(bpy)(CO2)]0. (S) + e- + H+ → [Ru-(trpy)(bpy)(COOH)]0. (D). -0.37. [Ru-(trpy)(bpy)(CO2)]1-. (D) + e- + H+ → [Ru-(trpy)(bpy)(COOH)]1-. (T). -0.51. [Ru-(trpy)(bpy)(CO2)]2-. (T) + e- + H+ → [Ru-(trpy)(bpy)(COOH)]2-. (D). -0.81. [Ru-(trpy)(bpy)(CO2)]0. (S). + H+ → [Ru-(trpy)(bpy)(COOH)]1+. (S). 28.5. [Ru-(trpy)(bpy)(CO2)]1-. (D). + H+ → [Ru-(trpy)(bpy)(COOH)]0. (D). 29.9. [Ru-(trpy)(bpy)(CO2)]2-. (T). + H+ → [Ru-(trpy)(bpy)(COOH)]1-. (T). 33.7. [Ru-(trpy)(bpy)(CO2)]3-. (D). + H+ → [Ru-(trpy)(bpy)(COOH)]2-. (D). 34.5. Table 3-2-3-1. Calculated values of pK and E0 at the Density Functional Theory Level of [Ru(bpy)(trpy)CH2OH] to [Ru(bpy)(trpy)COOH].. 25.

(35) Reaction. Gibbs free energy Spin. Spin. [Ru-(trpy)(bpy)(CH2OH)]0+ (D). + [EtOH] → CH3OH + [Ru-(trpy)(bpy)(OEt)]0 (D). 0.94. [Ru-(trpy)(bpy)(CH2OH)]1- (T). + [EtOH] → CH3OH + [Ru-(trpy)(bpy)(OEt)]1- (S). 3. [Ru-(trpy)(bpy)(CH2OH)]1- (T). + [EtOH] → CH3OH + [Ru-(trpy)(bpy)(OEt)]1- (T). 3.14. [Ru-(trpy)(bpy)(CH2OH)]2- (D). + [EtOH] → CH3OH + [Ru-(trpy)(bpy)(OEt)]2- (D). -18.44. [Ru-(trpy)(bpy)(CH2OH)]3- (T). + [EtOH] → CH3OH + [Ru-(trpy)(bpy)(OEt)]3- (S). -19.4. [Ru-(trpy)(bpy)(CH2OH)]3- (T). + [EtOH] → CH3OH + [Ru-(trpy)(bpy)(OEt)]3- (T). -19.97. [Ru-(trpy)(bpy)(OEt)]0. (D). +. CO2. → OEt- + [Ru-(trpy)(bpy)(CO2)]1+ (D). 15.85. [Ru-(trpy)(bpy)(OEt)]1-. (S). +. CO2. → OEt- + [Ru-(trpy)(bpy)(CO2)]0 (S). -2.48. [Ru-(trpy)(bpy)(OEt)]2-. (D). +. CO2. → OEt- + [Ru-(trpy)(bpy)(CO2)]1- (D). 4.77. [Ru-(trpy)(bpy)(OEt)]3-. (T). +. CO2. → OEt- + [Ru-(trpy)(bpy)(CO2)]2- (T). -12.4. Table 3-2-3-2. Calculated values of Gibbs free energy at the Density Functional Theory Level of [Ru(bpy)(trpy)CH2OH] to [Ru(bpy)(trpy)CO2].. 26.

(36) 3-2-4. The mechanism of [Ru(bpy)(trpy)CO]2+ The following Scheme 3-2-4. shows the mechanism of [Ru(bpy)(trpy)CO]2+.. Scheme 3-2-4. The mechanism of [Ru(bpy)(trpy)CO]2+. 27.

(37) 3-3. The reaction of [Ru(trpy)(OBQ)CO]2+ (Non-Innocent Ligand) 3-3-1. [Ru(trpy)(OBQ)CO] to [Ru(trpy)(OBQ)CHO]. Scheme 3-3-1. Calculated reduction potentials for converting [Ru(trpy)(OBQ)CO] to [Ru(trpy)(OBQ)CHO]. The green values represent the potentials for ET or PCET, and red values represent the pKa for PT.. If the pKa of a species is higher than that of the solvent, it can extract a proton from the solvent. Therefore, if there is a low-energy PCET pathway but the associated pKa is lower than that of the solvent, the pathway is impossible to execute. Accordingly, [Ru-CO]2+ does not undergo PCET pathways because of its high reduction potential energy (−2.11V) and low pKa (−43.2). It accepts an electron to form [Ru-CO]1+ with low potential energy, which undergoes a direct four-electron reduction to yield [Ru-CO]2-. Fortunately, the pKa (31.8) is higher than that of the solvent when generating [Ru-CO]2-; thus, it might carry out the PCET process to form [Ru-CHO]2- and the associated potential energy is only −1.04V. The other pathway is disproportionation of [Ru-CO]2- to form [Ru-CHO]2- and [Ru-CO]1-.. 28.

(38) Reactions. ET Spin. Spin. PCET. PT. potential potential pKa. [Ru-(trpy)(Obq)(CO)]2+. (S). +. e-. →. [Ru-(trpy)(Obq)(CO)]1+. (D). 1.08. [Ru-(trpy)(Obq)(CO)]1+. (D) +. e-. →. [Ru-(trpy)(Obq)(CO)]0+. (S). 0.03. [Ru-(trpy)(Obq)(CO)]1+. (D) +. e-. →. [Ru-(trpy)(Obq)(CO)]0+. (T). -0.84. [Ru-(trpy)(Obq)(CO)]0+. (S). +. e-. →. [Ru-(trpy)(Obq)(CO)]1-. (D). -1.08. [Ru-(trpy)(Obq)(CO)]1-. (D) +. e-. →. [Ru-(trpy)(Obq)(CO)]2-. (S). -1.80. [Ru-(trpy)(Obq)(CO)]1-. (D) +. e-. →. [Ru-(trpy)(Obq)(CO)]2-. (T). -1.71. [Ru-(trpy)(Obq)(CO)]2-. (S). +. e-. →. [Ru-(trpy)(Obq)(CO)]3-. (D). -2.20. [Ru-(trpy)(Obq)(CHO)]2+ (D) +. e-. →. [Ru-(trpy)(Obq)(CHO)]1+. (S). 1.11. [Ru-(trpy)(Obq)(CHO)]2+ (D) +. e-. →. [Ru-(trpy)(Obq)(CHO)]1+. (T). 1.07. [Ru-(trpy)(Obq)(CHO)]1+ (S). +. e-. →. [Ru-(trpy)(Obq)(CHO)]0+. (D). 0.46. [Ru-(trpy)(Obq)(CHO)]0+ (D) +. e-. →. [Ru-(trpy)(Obq)(CHO)]1-. (S). -0.60. [Ru-(trpy)(Obq)(CHO)]0+ (D) +. e-. →. [Ru-(trpy)(Obq)(CHO)]1-. (T). -1.18. [Ru-(trpy)(Obq)(CHO)]1- (S). +. e-. →. [Ru-(trpy)(Obq)(CHO)]2-. (D). -1.48. [Ru-(trpy)(Obq)(CHO)]2- (D) +. e-. →. [Ru-(trpy)(Obq)(CHO)]3-. (S). -2.24. [Ru-(trpy)(Obq)(CHO)]2- (D) +. e-. →. [Ru-(trpy)(Obq)(CHO)]3-. (T). -2.54. [Ru-(trpy)(Obq)(CO)]2+. (S). +. e- + H+ →. [Ru-(trpy)(Obq)(CHO)]2+. (D). -2.11. [Ru-(trpy)(Obq)(CO)]1+. (D) +. e- + H+ →. [Ru-(trpy)(Obq)(CHO)]1+. (S). -2.08. [Ru-(trpy)(Obq)(CO)]1+. (D) +. e- + H+ →. [Ru-(trpy)(Obq)(CHO)]1+. (T). -2.12. [Ru-(trpy)(Obq)(CO)]0+. (S). +. e- + H+ →. [Ru-(trpy)(Obq)(CHO)]0+. (D). -1.69. [Ru-(trpy)(Obq)(CO)]1-. (D) +. e- + H+ →. [Ru-(trpy)(Obq)(CHO)]1-. (S). -1.25. [Ru-(trpy)(Obq)(CO)]1-. (D) +. e- + H+ →. [Ru-(trpy)(Obq)(CHO)]1-. (T). -1.78. [Ru-(trpy)(Obq)(CO)]2-. (T). +. e- + H+ →. [Ru-(trpy)(Obq)(CHO)]2-. (D). -1.04. [Ru-(trpy)(Obq)(CO)]3-. (D) +. e- + H+ →. [Ru-(trpy)(Obq)(CHO)]3-. (S). -1.08. [Ru-(trpy)(bpy)(CO)]3-. (D) +. e- + H+ →. [Ru-(trpy)(Obq)(CHO)]3-. (T). -1.35. [Ru-(trpy)(Obq)(CO)]1+. (D). + H+ →. [Ru-(trpy)(Obq)(CHO)]2+. (D). -43.2. [Ru-(trpy)(Obq)(CO)]0+. (S). + H+ →. [Ru-(trpy)(Obq)(CHO)]1+. (S). -25.0. [Ru-(trpy)(Obq)(CO)]0+. (S). + H+ →. [Ru-(trpy)(Obq)(CHO)]1+. (T). -10.2. [Ru-(trpy)(Obq)(CO)]1-. (D). + H+ →. [Ru-(trpy)(Obq)(CHO)]0+. (D). 1.0. [Ru-(trpy)(Obq)(CO)]2-. (S). + H+ →. [Ru-(trpy)(Obq)(CHO)]1-. (S). 21.3. [Ru-(trpy)(Obq)(CO)]2-. (S). + H+ →. [Ru-(trpy)(Obq)(CHO)]1-. (T). 19.6. [Ru-(trpy)(Obq)(CO)]3-. (D). + H+ →. [Ru-(trpy)(Obq)(CHO)]2-. (D). 31.8. Table 3-3-1. Calculated values of pK and E0 at the Density Functional Theory Level of [Ru(trpy)(OBQ)CO] to [Ru(trpy)(OBQ)CHO].. 29.

(39) 3-3-2. [Ru(trpy)(OBQ)CHO] to [Ru(trpy)(OBQ)CH2OH]. Scheme 3-3-2. Calculated reduction potentials for converting [Ru(trpy)(OBQ)CHO] to [Ru(trpy)(OBQ)CH2OH]. The green and blue values represent the potentials for ET or PCET, and red values represent the pKa for PT.. There are two PCET pathways for [Ru-CHO]2-, one involving proton capture on carbon to form [Ru-CH2O]2- and the other proton transfer to the oxygen [Ruα. CH OH]2-. It can undergo PCET to form [Ru-CH2O]2-, but cannot form [RuCHOH]2-, because of the respective pKa values (31.7 and 22.2). As-formed [Ruα. β. CH2O]2- probably undergoes a further PCET step to generate [Ru-CH2 OH ]2-. In these two PCET pathways, the potential energy does not exceed −1.70V, making the processes feasible.. 30.

(40) Reactions. ET Spin. PCET. PT. Spin potential potential pKa. [Ru-(trpy)(Obq)(CH2OH)]1+. (T) + e-. → [Ru-(trpy)(Obq)(CH2OH)]0+. (D). 0.37. [Ru-(trpy)(Obq)(CH2OH)]0+. (D) + e-. → [Ru-(trpy)(Obq)(CH2OH)]1-. (S). -0.59. [Ru-(trpy)(Obq)(CH2OH)]0+. (D) + e-. → [Ru-(trpy)(Obq)(CH2OH)]1-. (T). -1.30. [Ru-(trpy)(Obq)(CH2OH)]1-. (S) + e-. → [Ru-(trpy)(Obq)(CH2OH)]2-. (D). -2.58. [Ru-(trpy)(Obq)(CH2OH)]2-. (D) + e-. → [Ru-(trpy)(Obq)(CH2OH)]3-. (S). -2.26. [Ru-(trpy)(Obq)(CH2OH)]2-. (D) + e-. → [Ru-(trpy)(Obq)(CH2OH)]3-. (T). -2.29. [Ru-(trpy)(Obq)(CHO)]1+. (S) + e- + H+ → [Ru-(trpy)(Obq)(CH2O)]1+. (D). -0.57. [Ru-(trpy)(Obq)(CHO)]1+. (S) + e- + H+ → [Ru-(trpy)(Obq)(CHOH)]1+. (D). -0.22. [Ru-(trpy)(Obq)(CHO)]0+. (D) + e- + H+ → [Ru-(trpy)(Obq)(CH2O)]0+. (S). -0.94. [Ru-(trpy)(Obq)(CHO)]0+. (D) + e- + H+ → [Ru-(trpy)(Obq)(CH2O)]0+. (T). -1.43. [Ru-(trpy)(Obq)(CHO)]0+. (D) + e- + H+ → [Ru-(trpy)(Obq)(CHOH)]0+. (S). -0.80. [Ru-(trpy)(Obq)(CHO)]0+. (D) + e- + H+ → [Ru-(trpy)(Obq)(CHOH)]0+. (T). -1.54. [Ru-(trpy)(Obq)(CHO)]1-. (S) + e- + H+ → [Ru-(trpy)(Obq)(CH2O)]1-. (D). -0.91. [Ru-(trpy)(Obq)(CHO)]1-. (S) + e- + H+ → [Ru-(trpy)(Obq)(CHOH)]1-. (D). -1.37. [Ru-(trpy)(Obq)(CHO)]2-. (D) + e- + H+ → [Ru-(trpy)(Obq)(CH2O)]2-. (S). -1.08. [Ru-(trpy)(Obq)(CHO)]2-. (D) + e- + H+ → [Ru-(trpy)(Obq)(CH2O)]2-. (T). -2.26. [Ru-(trpy)(Obq)(CHO)]2-. (D) + e- + H+ → [Ru-(trpy)(Obq)(CHOH)]2-. (S). -1.61. [Ru-(trpy)(Obq)(CHO)]2-. (D) + e- + H+ → [Ru-(trpy)(Obq)(CHOH)]2-. (T). -1.70. [Ru-(trpy)(Obq)(CHO)]3-. (S) + e- + H+ → [Ru-(trpy)(Obq)(CH2O)]3-. (D). -2.17. [Ru-(trpy)(Obq)(CHO)]3-. (S) + e- + H+ → [Ru-(trpy)(Obq)(CHOH)]3-. (D). -1.59. [Ru-(trpy)(Obq)(CH2O)]1+. (D) + e- + H+ → [Ru-(trpy)(Obq)(CH2OH)]1+. (T). -0.73. [Ru-(trpy)(Obq)(CHOH)]1+. (D) + e- + H+ → [Ru-(trpy)(Obq)(CH2OH)]1+. (T). -1.08. [Ru-(trpy)(Obq)(CH2O)]0+. (S) + e- + H+ → [Ru-(trpy)(Obq)(CH2OH)]0+. (D). -0.45. [Ru-(trpy)(Obq)(CHOH)]0+. (T) + e- + H+ → [Ru-(trpy)(Obq)(CH2OH)]0+. (D). -0.59. [Ru-(trpy)(Obq)(CH2O)]1-. (D) + e- + H+ → [Ru-(trpy)(Obq)(CH2OH)]1-. (S). -0.45. [Ru-(trpy)(Obq)(CHOH)]1-. (D) + e- + H+ → [Ru-(trpy)(Obq)(CH2OH)]1-. (S). 0. [Ru-(trpy)(Obq)(CH2O)]2-. (S) + e- + H+ → [Ru-(trpy)(Obq)(CH2OH)]2-. (D). -1.29. [Ru-(trpy)(Obq)(CHOH)]2-. (S) + e- + H+ → [Ru-(trpy)(Obq)(CH2OH)]2-. (D). -0.77. [Ru-(trpy)(Obq)(CH2O)]3-. (D) + e- + H+ → [Ru-(trpy)(Obq)(CH2OH)]3-. (S). -0.23. [Ru-(trpy)(Obq)(CHOH)]3-. (D) + e- + H+ → [Ru-(trpy)(Obq)(CH2OH)]3-. (S). -0.81. [Ru-(trpy)(Obq)(CHO)]0+. (D). + H+ → [Ru-(trpy)(Obq)(CH2O)]1+. (D). -4.4. [Ru-(trpy)(Obq)(CHO)]0+. (D). + H+ → [Ru-(trpy)(Obq)(CHOH)]1+. (D). 1.8. 31.

(41) [Ru-(trpy)(Obq)(CHO)]1-. (S). + H+ → [Ru-(trpy)(Obq)(CH2O)]0+. (S). 6.7. [Ru-(trpy)(Obq)(CHO)]1-. (S). + H+ → [Ru-(trpy)(Obq)(CH2O)]0+. (T). -2.3. [Ru-(trpy)(Obq)(CHO)]1-. (S). + H+ → [Ru-(trpy)(Obq)(CHOH)]0+. (S). 9.3. (T). -4.4. H+. →. [Ru-(trpy)(Obq)(CHO)]1-. [Ru-(trpy)(Obq)(CHOH)]0+. (S). +. [Ru-(trpy)(Obq)(CHO)]2-. (D). + H+ → [Ru-(trpy)(Obq)(CH2O)]1-. (D). 22. [Ru-(trpy)(Obq)(CHO)]2-. (D). + H+ → [Ru-(trpy)(Obq)(CHOH)]1-. (D). 13.7. [Ru-(trpy)(Obq)(CHO)]3-. (S). + H+ → [Ru-(trpy)(Obq)(CH2O)]2-. (S). 31.7. [Ru-(trpy)(Obq)(CHO)]3-. (S). + H+ → [Ru-(trpy)(Obq)(CH2O)]2-. (T). 10.1. [Ru-(trpy)(Obq)(CHO)]3-. (S). + H+ → [Ru-(trpy)(Obq)(CHOH)]2-. (S). 22.2. [Ru-(trpy)(Obq)(CHO)]3-. (S). + H+ → [Ru-(trpy)(Obq)(CHOH)]2-. (T). 20.4. [Ru-(trpy)(Obq)(CHO)]4-. (D). + H+ → [Ru-(trpy)(Obq)(CH2O)]3-. (D). 17.6. [Ru-(trpy)(Obq)(CHO)]4-. (D). + H+ → [Ru-(trpy)(Obq)(CHOH)]3-. (D). 28.2. [Ru-(trpy)(Obq)(CH2O)]0+. (S). + H+ → [Ru-(trpy)(Obq)(CH2OH)]1+. (T). -0.7. [Ru-(trpy)(Obq)(CHOH)]0+. (S). + H+ → [Ru-(trpy)(Obq)(CH2OH)]1+. (T). 10.4. [Ru-(trpy)(Obq)(CH2O)]1-. (D). + H+ → [Ru-(trpy)(Obq)(CH2OH)]0+. (D). 15.3. [Ru-(trpy)(Obq)(CHOH)]1-. (D). + H+ → [Ru-(trpy)(Obq)(CH2OH)]0+. (D). 23.6. [Ru-(trpy)(Obq)(CH2O)]2-. (S). + H+ → [Ru-(trpy)(Obq)(CH2OH)]1-. (S). 33.5. [Ru-(trpy)(Obq)(CHOH)]2-. (S). + H+ → [Ru-(trpy)(Obq)(CH2OH)]1-. (S). 43. [Ru-(trpy)(Obq)(CH2O)]3-. (D). + H+ → [Ru-(trpy)(Obq)(CH2OH)]2-. (D). 47.7. [Ru-(trpy)(Obq)(CHOH)]3-. (D). + H+ → [Ru-(trpy)(Obq)(CH2OH)]2-. (D). 37.1. Table 3-3-2. Calculated values of pK and E0 at the Density Functional Theory Level of [Ru(trpy)(OBQ)CHO] to [Ru(trpy)(OBQ)CH2OH].. 32.

(42) 3-3-3. [Ru(trpy)(OBQ)CH2OH] to [Ru(trpy)(OBQ)CO]. Scheme 3-3-3. Calculated reduction potentials for converting [Ru(trpy)(OBQ)CH2OH] to [Ru(trpy)(OBQ)CO]. The green values represent the potentials for ET or PCET, red values represent the pKa for PT, and blue values represent for the gibbs energy.. [Ru-CH2αOHβ]2- can accept a proton to remove CH3OH, and the resultant vacancy will be instantaneously replaced by ethanol or CO2 because the Gibbs free energy of the former is negative (−34.13 kcal/mol) and that of the latter is positive (21.22 kcal/mol). This means that the former reaction is spontaneous, but the latter reaction can be achieved only by supplying more energy. In contrast to [Ru-(trpy)(bpy)OEt]2-, the vacancy of proton exaction from [Ru-(trpy)(Obq)OEt]2- is hard to replace by CO2. It considers that a non-innocent ligand as a good π -acceptor ligand with a mild potential. The stronger π-back donation should make the charge density retain in the ligand, which make Ru less electron rich and inhibit CO2 coordination.30 The intermediate is [Ru-CO2]1- that undergoes PCET to form [Ru-COOH]1- with 33.

(43) low potential energy (−0.15V) and suitable pKa (33.7) or undergoes ET to form [RuCO2]2- with potential (-1.35V) then undergoes PCET to form [Ru-COOH]2-. In the final step, the dehydration from [Ru-COOH]2- to form [Ru-CO]1-, which carries on performing all the above-mentioned reactions.. 34.

(44) Reactions. ET Spin. PCET. PT. Spin potential potential pKa. [Ru-(trpy)(Obq)(OEt)]0. (D) + e-. → [Ru-(trpy)(Obq)(OEt)]1-. (S). -0.51. [Ru-(trpy)(Obq)(OEt)]0. (D) + e-. → [Ru-(trpy)(Obq)(OEt)]1-. (T). -1.21. [Ru-(trpy)(Obq)(OEt)]1-. (S) + e-. → [Ru-(trpy)(Obq)(OEt)]2-. (D). -1.67. [Ru-(trpy)(Obq)(OEt)]2-. (D) + e-. → [Ru-(trpy)(Obq)(OEt)]3-. (S). -2.34. [Ru-(trpy)(Obq)(OEt)]2-. (D) +. e-. →. [Ru-(trpy)(Obq)(OEt)]3-. (T). -2.18. [Ru-(trpy)(Obq)(CO2)]1+. (D) + e-. → [Ru-(trpy)(Obq)(CO2)]0. (S). -0.19. [Ru-(trpy)(Obq)(CO2)]1+. (D) + e-. → [Ru-(trpy)(Obq)(CO2)]0. (T). -0.55. [Ru-(trpy)(Obq)(CO2)]0. (S) + e-. → [Ru-(trpy)(Obq)(CO2)]1-. (D). -1.04. [Ru-(trpy)(Obq)(CO2)]1-. (D) + e-. → [Ru-(trpy)(Obq)(CO2)]2-. (S). -1.35. [Ru-(trpy)(Obq)(CO2)]1-. (D) + e-. → [Ru-(trpy)(Obq)(CO2)]2-. (T). -1.63. [Ru-(trpy)(Obq)(COOH)]1+. (S) + e-. → [Ru-(trpy)(Obq)(COOH)]0. (D). 0.50. →. [Ru-(trpy)(Obq)(COOH)]1-. (S). -0.61. [Ru-(trpy)(Obq)(COOH)]0. e-. (D) +. [Ru-(trpy)(Obq)(COOH)]0. (D) + e-. → [Ru-(trpy)(Obq)(COOH)]1-. (T). -1.28. [Ru-(trpy)(Obq)(COOH)]1-. (T) + e-. → [Ru-(trpy)(Obq)(COOH)]2-. (D). -1.50. [Ru-(trpy)(Obq)(CO2)]1+. (D) + e- + H+ → [Ru-(trpy)(Obq)(COOH)]1+. (S). -1.20. [Ru-(trpy)(Obq)(CO2)]0. (S) + e- + H+ → [Ru-(trpy)(Obq)(COOH)]0. (D). -0.56. [Ru-(trpy)(Obq)(CO2)]1-. (D) + e- + H+ → [Ru-(trpy)(Obq)(COOH)]1-. (T). -0.15. [Ru-(trpy)(Obq)(CO2)]2-. (T) + e- + H+ → [Ru-(trpy)(Obq)(COOH)]2-. (D). -0.29. [Ru-(trpy)(Obq)(CO2)]0. (S). + H+ → [Ru-(trpy)(Obq)(COOH)]1+. (S). -5.0. [Ru-(trpy)(Obq)(CO2)]1-. (D). + H+ → [Ru-(trpy)(Obq)(COOH)]0. (D). 21.1. [Ru-(trpy)(Obq)(CO2)]2-. (T). + H+ → [Ru-(trpy)(Obq)(COOH)]1-. (T). 33.7. [Ru-(trpy)(Obq)(CO2)]3-. (D). + H+ → [Ru-(trpy)(Obq)(COOH)]2-. (D). 34.3. Table 3-3-3-1. Calculated values of pK and E0 at the Density Functional Theory Level of [Ru(trpy)(OBQ)CH2OH] to [Ru(trpy)(OBQ)COOH].. 35.

(45) Reaction. Gibbs free energy. [Ru-(trpy)(Obq)(CH2OH)]0+. (D) + [EtOH] → CH3OH + [Ru-(trpy)(Obq)(OEt)]0. (D). -11.47. [Ru-(trpy)(Obq)(CH2OH)]1-. (T) + [EtOH] → CH3OH + [Ru-(trpy)(Obq)(OEt)]1-. (S). -13.20. [Ru-(trpy)(Obq)(CH2OH)]1-. (T) + [EtOH] → CH3OH + [Ru-(trpy)(Obq)(OEt)]1-. (T). 2.81. [Ru-(trpy)(Obq)(CH2OH)]2-. (D) + [EtOH] → CH3OH + [Ru-(trpy)(Obq)(OEt)]2-. (D). -34.13. [Ru-(trpy)(Obq)(CH2OH)]3-. (T) + [EtOH] → CH3OH + [Ru-(trpy)(Obq)(OEt)]3-. (S). -32.45. [Ru-(trpy)(Obq)(CH2OH)]3-. (T) + [EtOH] → CH3OH + [Ru-(trpy)(Obq)(OEt)]3-. (T). -36.13. [Ru-(trpy)(Obq)(OEt)]0. (D) + CO2. → OEt-. + [Ru-(trpy)(Obq)(CO2)]1+. (D). 43.18. [Ru-(trpy)(Obq)(OEt)]1-. (S) + CO2. → OEt-. + [Ru-(trpy)(Obq)(CO2)]0. (S). 35.69. [Ru-(trpy)(Obq)(OEt)]2-. (D) + CO2. → OEt-. + [Ru-(trpy)(Obq)(CO2)]1-. (D). 21.22. [Ru-(trpy)(Obq)(OEt)]3-. (T) + CO2. → OEt-. + [Ru-(trpy)(Obq)(CO2)]2-. (S). 2.25. Table 3-3-3-2. Calculated values of Gibbs free energy at the Density Functional Theory Level of [Ru(trpy)(OBQ)CH2OH] to [Ru(trpy)(OBQ)CO2].. 36.

(46) 3-3-4. The mechanism of [Ru(trpy)(OBQ)CO]2+ The following Scheme 3-3-4. shows the mechanism of [Ru(trpy)(OBQ)CO]2+.. Scheme 3-3-4. The mechanism of [Ru(trpy)(OBQ)CO]2+. 37.

(47) 3-4. Charge and Spin distribution From the electron distribution in each orbital, the data of charge and spin density are known to be reduced to which part of the metal complexes, and the electron configurations of the complexes of this metal are predictable. The complexes discussed are divided into four structural parts: metal (Ru), bidentate ligand (bpy), tridentate ligand (trpy), and monodentate ligands (CO derivatives).. 3-4-1. [Ru(bpy)(trpy)CO]2+ Charge Complex. Spin. [Ru-CO]2+ [Ru-CO]1+. Charge distribution Ru. Bpy. Trpy. CO. Singlet. 1.44. 0.21. 0.31. 0.05. Doublet. 1.41. -0.07. -0.34. 0. [Ru-CO]. Triplet. 1.4. -0.57. -0.77. -0.06. [Ru-CO]1-. Doublet. 1.36. -0.79. -1.47. -0.1. [Ru-CHO]1-. Triplet. 1.31. -0.73. -1.24. -0.34. Doublet. 1.37. -0.85. -1.34. -0.17. Triplet. 1.47. -0.8. -1.32. -0.35. [Ru-OEt]. Singlet. 1.67. -0.87. -1.19. -0.62. [Ru-OEt]2-. Doublet. 1.61. -1.17. -1.78. -0.65. [Ru-CO2]1-. Doublet. 1.31. -0.71. -1.14. -0.46. [Ru-CO2]. Triplet. 1.2. -0.96. -1.61. -0.62. [Ru-COOH]2-. Doublet. 1.34. -1.12. -1.77. -0.46. 0. [Ru-CH2O]1[Ru-CH2OH]. 1-. 1-. 2-. Table 3-4-1-1. Charge distribution of [Ru(bpy)(trpy)CO]2+. 38.

(48) Spin Spin distribution. Complex. Spin. [Ru-CO]2+. Singlet. [Ru-CO]1+. Doublet. 0.03. 0.24. 0.73. 0.01. Triplet. 0.05. 0.81. 1.12. 0.02. Doublet. -0.01. -0.94. 1.94. 0.01. Triplet. 0.13. 0.73. 1.15. 0. Doublet. 0.09. 0.88. 0.04. 0. 0.18. 0.78. 1.05. 0. 0. [Ru-CO]. [Ru-CO]11-. [Ru-CHO]. 1-. [Ru-CH2O]. [Ru-CH2OH]1- Triplet [Ru-OEt]1-. Ru. Bpy. Trpy. CO. Singlet. 2-. [Ru-OEt]. Doublet. 0.05. -0.62. 1.57. 0. [Ru-CO2]1-. Doublet. -0.02. 0.71. 0.32. 0. Triplet. 0.09. 0.9. 1.04. -0.03. Doublet. 0.03. -0.69. 1.66. 0. [Ru-CO2]22-. [Ru-COOH]. Table 3-4-1-2. Spin distribution of [Ru(bpy)(trpy)CO]2+ According to the above Tables 3-4-1-1 and 3-4-1-2, the distributions of charge and spin do not change visibly over Ru. From [Ru-CO]2+ to [Ru-CO]1-, the charge density decreases only slightly in the CO part, but bpy and trpy parts have significantly reduced charge distributions, that the electron distributions of 3 electrons in the trpy and bpy parts are 1.78 (0.31-(-1.47)) and 1 (0.21-(-0.79)). Otherwise, it reveals that the first electron is reduced on trpy, the second is on bpy, and the third is on trpy. When [Ru-CO]1- was converted to [Ru-CHO]1- by PCET, the charge and spin distributions of bpy remain unchanged; however, the charge density of trpy increases and its spin density decreases, implying that the trpy ligand is oxidized. The charge density of CHO decreased because in addition to undergoing an electron transfer and a protonation, trpy electron density is transferred to CHO as well. After the PCET process, [Ru-CHO]1- is converted to [Ru-CH2O]1-, increasing the charge distribution in CHO with part of its charge transfers to trpy. 39.

(49) When [Ru-CH2O]1- becomes [Ru-CH2OH]1- via PCET, charge density of [RuCH2OH]2- is slightly decreased. Charge density increases sharply on accepting a proton, but that did not happen in this case. A possible explanation is that electron transfer on CH2OH in order to receive a proton. When [Ru-CH2OH]1- gets converted to [Ru-OEt]1-, the charge distribution of trpy increases. This implies that after CH2OH obtained a proton from the solvent, OEt- binds to the complex and forms [Ru-OEt]1-, while trpy provides one electron to CH2OH. Scheme 3-2-3 shows that the Gibbs free energy is 3 kcal/mol when [Ru-CH2OH]1- is replaced by the solvent to form [Ru-OEt]1-. The conversion of [Ru-OEt]2- to [Ru-CO2]1- shows that the charge distributions of bpy and trpy have increasing tendencies but the spin distribution of trpy is decreased. Therefore, we can assume that trpy provides an electron to OEt- to abstract a proton that makes it back to the solvent. In the meantime, CO2 fills the cavity. As [Ru-CO2]1- is converted to [Ru-CO2]2- by one-electron reduction, the charge density is reduced in trpy apparently. In the end, when [Ru-CO2]2- undergoes PCET to form [Ru-COOH]2-, there is no obvious change in charge distribution but on the spin distribution in trpy is increasing. Accordingly, we can assume that H+ is reduced in CO2 and the electron is reduced in trpy. At this moment, COOH will obtain a proton and remove a H2O to form [Ru-CO]1and the charge on trpy will decrease again. Thus, we can assume that trpy provides electron to COOH, which then obtains a proton and undergoes dehydration. Above all, this reaction reduces two electrons reduced in trpy and one in bpy, following with a series of electron transfer on trpy. In the end, when [Ru-CO2]1- is formed, trpy has two electrons and bpy has one electron.. 40.

(50) 3-4-2. [Ru(trpy)(OBQ)CO]2+ Charge Complex. Spin. [Ru-CO]2+. Charge distribution Ru. OBQ. Trpy. CO. Singlet. 1.42. 0.02. 0.48. 0.09. [Ru-CO]1+. Doublet. 1.38. -0.57. 0.2. 0. [Ru-CO]0. Singlet. 1.34. -1.07. -0.19. -0.09. [Ru-CO]. Doublet. 1.29. -1.26. -0.87. -0.16. [Ru-CO]2-. Singlet. 1.2. -1.38. -1.6. -0.22. Doublet. 1.26. -1.39. -1.48. -0.38. [Ru-CH2O]. Singlet. 1.38. -1.36. -1.61. -0.42. [Ru-CH2OH]2-. Doublet. 1.18. -1.33. -1.55. -0.3. [Ru-OEt]2-. Doublet. 1.52. -1.38. -1.53. -0.62. [Ru-CO2]1-. Doublet. 1.38. -1.22. -0.87. -0.3. Singlet. 1.34. -1.31. -1.58. -0.46. Doublet. 1.24. -1.49. -1.38. -0.35. 1-. [Ru-CHO]22-. 2-. [Ru-CO2]. 2-. [Ru-COOH]. Table 3-4-2-1. Charge distribution of [Ru(trpy)(OBQ)CO]2+ Spin Complex. Spin. [Ru-CO]2+. Singlet. [Ru-CO]1+. Doublet. [Ru-CO]0. Singlet. Ru. Spin distribution OBQ Trpy. CO. 0.09. 0.91. 0. 0. 0.03. 0. 0.95. 0.02. -0.02. -0.01. 1.03. 0. Doublet. -0.07. -0.01. 1.08. 0. [Ru-OEt]2-. Doublet. -0.02. 0. 1.03. -0.01. [Ru-CO2]1-. Doublet. 0.05. 0.01. 0.88. 0.05. 0.04. 0.96. 0. 0. 1-. [Ru-CO]. Doublet. [Ru-CO]2-. Singlet. [Ru-CHO]2-. Doublet. 2-. [Ru-CH2O]. [Ru-CH2OH]. Singlet 2-. 2-. [Ru-CO2]. Singlet. [Ru-COOH]2-. Doublet. Table 3-4-2-2. Spin distribution of [Ru(trpy)(OBQ)CO]2+. 41.

(51) According to Tables 3-4-2-1 and 3-4-2-2, when [Ru-CO]2+ is reduced to [Ru-CO]2-, the charge and spin distributions of Ru decrease slightly, but the tendency is more obvious than [Ru(bpy)(trpy)CO]2+. The charge density is lowered slightly for CO too. For OBQ and trpy, the charge distribution decreases sharply, implying that four electrons are reduced in OBQ and trpy. Compared with [Ru(bpy)(trpy)CO]2+, [Ru(bpy)(OBQ)CO]2+ involves reduction of two electrons in OBQ first, followed by two electrons in trpy. It reveals that the charge density is more delocalized between the metal and the ligand, which means that it can carry out a reduction better. Among trpy and OBQ, the data shows trpy is 2.08 (0.48-(-1.6)) while obq is around 1.4 (0.02-(1.38)). In the next step, [Ru-CO]2- undergoes a PCET process to form [Ru-CHO]2- after trpy and OBQ accept two electrons each. When [Ru-CO]2- is converted to [Ru-CHO]2-, the charge distributions of trpy and CO decrease. If there is a proton transfer in CO, the charge density must increase; however, the charge density decreases in this case as well. According to this speculation, apart from one electron reduction in CO, the charge density of trpy is partly transferred to CO. When [Ru-CH2O]2- is formed, charge density of Ru increases, and that of trpy decreases. It reveals a proton-coupled electron transfer process occurs on CHO, and the charge of Ru is fed back to trpy. When [Ru-CH2O]2-is converted to [Ru-CH2OH]2-, charge density of Ru decreases. It reveals a proton-coupled electron transfer process occurs on CH2O. It’s worth mentioning that the charge density is delocalized between the metal and the ligand at the same time, which enhance the removal of methanol. When [Ru-OEt]2-is converted to [Ru-CO2]1-, the charge density of trpy increases; therefore, we can assume that trpy provides electrons to OEt- to obtain a proton, which 42.

(52) then makes it back to the solvent while CO2 fills the cavity simultaneously. As [Ru-CO2]1- is converted to [Ru-CO2]2- by one-electron reduction, the charge density is reduced in trpy apparently. As [Ru-CO2]2- is converted to [Ru-COOH]2- via PCET, the charge and spin distributions of each part do not change much. The obtained data suggest that PCET occurs on CO2. If COOH obtains a proton and loses a H2O to form [Ru-CO]1-, the charge density of trpy increases again. We can still say that trpy provides electron to COOH to obtain proton and undergo dehydration. Overall, the first step is to reduce two electrons each in OBQ and in trpy; next, OBQ provides no electrons to the reaction, but trpy provides one electron to reduce CO2. When [Ru-CO2]1- is formed, two electrons are reduced in OBQ, and one electron is reduced in trpy.. 43.

(53) Chapter 4 Conclusion We successfully predicted the mechanism of CO2 reduction with two different types of ligands (innocent and non-innocent). The reactivities of the Ru complexes are different, and the CO2 reduction reaction was catalyzed following two different pathways. Although o-Benzoquinone (OBQ), a non-innocent ligand, does not reduce to a CO derivative, the electron density of [Ru(trpy)(OBQ)(CO)]2+ is more delocalized between the metal and the ligand, forming a nice low-potential pathway in contrast to [Ru(trpy)(bpy)(CO)]2+. The more electrons delocalized between metal and ligand, the more ability of reducing CO of the complex first. It reveals that OBQ system has more ability than bpy system of electrons reduced at the same condition. Therefore, a noninnocent ligand has the advantage of reducing CO to form methanol easily on the complex, but the CO2 activation after removal of methanol on ethanol is weaker than innocent ligand system (bpy). In contrast to non-innocent ligand system (OBQ), innocent ligand has less ability from reducing CO to methanol of the complex, but once methanol is removed, CO2 is easy to coordinate with Ru. Obviously, the dramatic difference of mechanism between two different ligands coordinate on Ru resulted in different pathways.. 44.

(54) References (1.) National Oceanic & Atmospheric Administration (2016). Full Mauna Loa CO2 record. Retrieved from http://www.esrl.noaa.gov/gmd/ccgg/trends/full.html (2.) Yasuda, H.; Bruckmeier, C.; Riege, B.; A, W.; Herrmann; Kuhn, F. E. Angew. Chem. Int. Ed. 2011, 50, 8510. (3.) Roy, L.; Zimmerman, P. M.; Paul, A.; Chem. Eur. J. 2011, 17, 435 (4.) Methanol Institute (2011). Methanol: The Clear Transportation. Retrieve from http://www.methanol.org/. Alternative. for. (5.) I. Ganesh; Renew. & Sust. Energ. Rev, 2014, 31, 221 (6.) Krebs, F.C.; Mikkelsen, M. Energy Environ. Sci. Energy Environ. Sci., 2010, 3, 43 (7.) Sakakura, T; C, J-C.; and Yasuda, H.; Chem. Rev. 2007, 107, 2365 (8.) Xu Xiaoding; J. A. Moulijn; Energy & Fuels, 1996, 10, 305 (9.) Lehn, J-M. ; Ziessel, R; J. Organomet. Chem., 1990, 382, 157 (10.) Hawecker, J; Lehn, J.-M.; Ziessel, M; J. Chem. Soc., Chem. Commun., 1984, 328 (11.) Savéant, J.-M. Chem. Rev. 2008, 108, 2348. (12.) M. Jitaru, D. A. Lowy, M. Toma and L. Oniciu, J. Appl. Electrochem., 1997, 27, 875 (13.) G. A. Olah, A. Goeppert and G. K. S. Prakash, J. Org. Chem., 2009,74, 487. (14.) Caleb Stewart; Mir-Akbar Hessami; Energ. Convers. Manage., 2005,46, 403 (15.) M. C. J. Bradford and M. A. Vannice, Appl. Catal., A, 1996, 142, 73. (16.) D. J. Fauth, E. A. Frommell, J. S. Hoffman, R. P. Reasbeck and H. W. Pennline, Fuel Process. Technol., 2005, 86, 1503. (17.) Savéant, J.M.; Costentin, C.; J. Am. Chem. Soc. 2011, 133, 19160. (18.) Huynh, My Hang V.; Meyer, Thomas J.; Chemical Reviews,2007, 107, 5004 (19.) Miyazaki, S.; Kojima, T.; Mayer J.M.; Fukuzumi, S; J. AM. CHEM. SOC. 2009, 131, 11615 (20.) Wenger, O.S. Acc. Chem. Res., 2013, 7, 1517 (21.) HAMMARSTRÖ M, L; MAGNUSON, A; ANDERLUND, M; Acc. Chem. 45.

(55) Res. 2009, 42, 1899 (22.) Meyer, T. J.; Acc. Chem. Res. 1989, 22,163 (23.) Sutin, N.; Marcus, R.A.; Biochem Biophys. Acta. 1985, 811, 265 (24.) Warren, J. J. ; Tronic, T. A. ; Mayer, J. M. ; Chem. Rev. 2010, 110, 6961 (25.) Benson, E. E.; Kubiak, C. P.; Sathrum, A. J.; Smieja, J. M. Chem. Soc. Rev 2009, 38, 89. (26.) Tanaka, K. Chemistry Letters 1993, 955. (27.) Tanaka, K.; Nagao, H.; Mizukawa, T. Inorg. Chem. 1994, 33, 3415. (28.) Hoffman, M. Z.; D'Angelantonio, M.; Mulazzan, Q. G.; J. Phys. Chem. 1991, 95, 5121 (29.) Johnson, B.A.; Maji, S.; Ott, S; Angew. Chem. Int. Ed. 2016, 55, 1825 (30.) Pickup, P. G.; Begum, A.; Electrochem. Commun., 2007, 9, 2525 (31.) Huang, K. W.; Min, S; Rasul, S; ChemPlusChem., 2016, 81, 166 (32.) Kaim, W.; Schwederski, B; Coord. Chem. Rev.,2010, 254,1580 (33.) Boyer, J.L.; Rochford, J.; Tsai, M.-K.; Muckerman, J.T. Inorg. Chem. Rev. 2010, 254, 309 (34.) Rochford, J.; Tsai, M.-K.; Muckerman, J.T.; Fujita, E.; Inorg. Chem. 2010, 49, 860 (35.) Truhlar, D.G.; Marenich, A.V. Angew. Chem. Int. Ed. 2012, 51, 12810 (36.) Marenich, A. V.; Cramer,J. C.; Truhlar, D. G.; J. Phys. Chem. B, 2009, 113, 6378 (37.) Foresman, J. B.; Frisch, A. Exploring Chemistry with Electronic Structure Methods: A Guide to Using Gaissian; Second Edition ed. Pittsburgh, PA. (38.) Foresman, J. B. Exploring Chemistry with Electronic Structure Methods; 2th ed. Pittsburgh, 2000. (39.) Tsai, M.-K.; Rochford, J.; Polyansky, D. E.; Wada, T.; Tanaka, K.; Fujita, E.; Muckerman, J. T. Inorganic Chemistry 2009, 48, 4372. (40.) Sadlej-Sosnowska, N. Theor. Chem. Acc. 2007, 118, 281. 46.